This invention concerns weathering resistant steel materials and, it relates to a flow rust reducing weathering resistant steel materials capable of effectively reducing occurrence of flow rust in relatively less salty circumstances such as mountain districts, rural districts and industrial districts, as well as steel materials excellent in earthquake proofness and coast weathering resistance applicable as steel structures such as bridges used in salty circumstances such as coast districts. The weathering resistance referred to in the invention means weathering resistance in a case of use in atmospheric air of coast districts.
1) Less Salty Circumstance
Weathering resistant steels with improved weathering resistance in atmospheric air with addition of alloying elements such as P, Cu, Cr and Ni in the steels have been used generally for structures such as bridges. The weathering resistant steels form, in several years, rust referred to as stable rust less permeating oxygen and water causing corrosion and suppress subsequent corrosion. Accordingly, the weathering resistant steels require no coating of anti-rust paints and they are highly corrosion resistant material which can be used in a so-called naked state.
However, since as long as several years are required till the stable rust is formed during which flow rust occurs in the weathering resistant steels, they involve problems of deteriorating scenes and causing environmental contamination.
In view of the problems described above, Japanese Patent Laid-Open No. 136557/1994, for example, proposes a surface treating method for steel materials of coating an aqueous solution of chromium sulfate or an aqueous solution of copper sulfate and further applying organic resin coating after drying the water content. Further, Japanese Patent Laid-Open No. 13158/1996 proposes a surface treating method of steel materials of coating an aqueous solution containing aluminum ions and further forming an organic resin film after drying of the water content.
However, in the techniques described in Japanese Patent Laid-Open No. 136557/1994 and Japanese Patent Laid-Open No. 13158/1996, while stable rust is grown in a short period of time, they still leave problems such that steps are complicated and the surface treating agents used are expensive, and development of weathering resistant steel materials not requiring surface treatment have been demanded.
In view of the above, for coping with such a demand, the invention intends to provide flow rust reducing weathering resistant steel materials capable of reducing occurrence of flow rust in the course of forming stable rust in weathering resistant steels used in a naked state.
2) Salty Circumstance such as Coast Districts
Steel structures such as bridge girders are generally applied with corrosion preventive means such as coating since their service life is long. However, the coating films are degraded to gradually reduce the corrosion preventive effect by chalking due to UV-rays, or expansion of rust by the corrosion under coating films. Accordingly, re-coating has been obliged on every certain periods. However, shortening of coating operators and increase in the personal expense in recent years makes the re-coating operation difficult. In view of the situations, weathering resistant steels requiring no coating of anti-rust paints and usable in a naked state have been applied more and more in steel structures.
The weathering resistant steels are those steel materials with addition of P, Cu, Ni and Cr in which stable rusts as protective films are formed in several years on the surface of steels in an atmospheric circumstance. Since the stable rust suppresses further development of corrosion, corrosion of the steel materials can be minimized. Accordingly, most of them are used with no coating.
However, in salty circumstances such as coast districts, no stable salt is formed after lapse of several years even in weathering resistant steels and steel materials are attacked violently.
In recent years, application guideline for weathering resistant steels have been issued from Minister of Construction (Joint Research Report Regarding Application of Weather Resistant Steel Material to Bridges (XX), March 1993, published from Civil Engineering Institute of Minister of Construction, KOZAI CLUB Co. and Nippon Kyoryo Kensetsu Kyokai), in which it is specified that existent weathering resistant steels (JIS G 3114: weathering resistant hot rolled steel materials for welding structure) can not be used with no coating in the district where atmospheric salt content is 0.05 mg/dm2/day or more, that is, in coast districts.
Accordingly, in salty circumstances such as coast districts, countermeasure has been adopted by applying coating such as of phthalic acid resin, chlorinated rubber or tar epoxy resin to ordinary steel materials. However, since bridges constructed in coast districts near the estuaries are often long and large and the corrosion is violent because of the use in the coast districts, the re-coating operation is extremely difficult and, accordingly, there is a strong demand for the steel materials that can be used with no coating.
Regarding this Japanese Patent Laid-Open No. 136557/1994, for example, leaves problems as described above.
Further, Japanese Patent No. 2572447, Japanese Patent Laid-Open NO. 51668/1993 and Japanese Patent Laid-Open No. 134587/1996 propose methods of improving the coast weathering resistance by adding a great amount of alloying elements such as P, Cu, Ni and Mo to steel materials.
However, referring to the bridge, the corrosive circumstance for steel materials are not always identical depending on the places to be used. Considering, for example, four main beam bridge, while outside of the beams are exposed to rainfall, water of condensation and sunshine, inside of the beam are exposed only to water of condensation but not suffer from rainfall. Generally, in a clean circumstance with no atmospheric salt content, it is said that the extent of corrosion is less in the inside of the beams when compared between the inside and the outside of the beams. On the other hand, in the circumstance with high atmospheric salt content, it is said that the extent of corrosion is rather greater in the inside of the beam than the outside of the beam. This reversal phenomenon occurs at a certain content of the atmospheric salt content as a boundary but the content can not be specified.
However, since outer beams, main beams and webs are exposed to two circumstances (with or without exposure to rainfalls) simultaneously (rear face and surface of plates), it is necessary for the steel materials to be used in steel structures such as bridges to maintain high weathering resistance in both of the circumstances.
However, in the existent techniques, evaluation was applied only under one circumstance (with rainfall or without rainfall), and development for steel materials having excellent coast weathering resistance simultaneously under two circumstances has been demanded.
3) Earthquake Proofness
On the other hand, the structural steel materials of this type utilized, for example, in bridge beams, have been (demanded to have an absorbed energy of 47J or more at xe2x88x925xc2x0 C. in a Charpy impact test in the rolling direction (L direction) and a cross direction (C direction) to the rolling direction of the steel materials in view of the safety. However, it has been found that high stresses may possibly exert in the direction of the plate thickness of the material to be used (Z direction) depending on the structure and the portions of the structures in large scale earthquakes such as Hanshin-Awaji disaster, so that it has been demanded for the steel materials for use in structures to improve the toughness in the direction of the plate thickness (Z direction) including the weld heat affect zone in order to further increase the earthquake proofness of steel materials after the Hanshin-Awaji disaster.
From the view points (1)-(3) above, the invention intends to provide a steel material capable of forming stable rust with good protective performance in relatively less salty districts and salty circumstance such as coast districts, regardless of rainfalls, excellent in weather proofness and excellent in earthquake proofness with improved toughness in the direction of Z also including the weld heat affective zone.
1) Flow Rust Reducing Weathering Resistant Steel Material
The present inventors have made an earnest study for the thickness capable of reducing flow rust in weathering resistant steels and, as a result, have found that a weathering resistant steel material capable of outstandingly reducing the amount of flow rust by adding B and, further, by controlling the content of B and the content of one or more of P, Cu, Ni, Cr and Mo based on a certain relationship to each other.
The invention has been achieved on the basis of this finding and the feature resides in a flow rust reducing weathering resistant steel material having a composition containing, on the weight % basis,
C: from 0.001% to 0.050%, Si: 0.60% or less, Mn: from 0.50% to 3.00%, S: 0.01% or less, Al: 0.10% or less and B: from 0.0003% to 0.0050% and, further, one or more of elements selected from P: from 0.005% to 0.15%, Cu; from 0.1% to 2.0%, Ni: from 0.1% to 6.0%, Cr: from 0.005% to 1.0% and Mo: from 0.005% to 1.0%, and satisfying the following equation (1):
(20P+3Cu+3Ni+6Cr+Mo)/(1xe2x88x920.2(10000B)0.4)xe2x89xa718xe2x80x83xe2x80x83(1) 
in which P, Cu, Ni, Cr, Mo, B: content for each element (wt %)), and the balance of Fe and inevitable impurities.
Further, in this invention, one or more of elements selected from Nb: from 0.005% to 0.20%, Ti: from 0.005% to 0.20%, V: from 0.005% to 0.20%, on the weight % basis, may be contained in addition to the composition described above.
Further, in the invention, one or more of elements selected from Ca: 0.02% or less and REM: 0.02% or less may be contained, on the weight % basis, in addition to the composition described above.
2) Coast Weathering Resistant Steel Material
The present inventors have made an earnest study for improving the coast weathering resistance and, as a result, have obtained a knowledge that Cr degrade the weathering resistance in circumstance containing much salt. Further, the present inventors have found that steel materials of excellent weathering resistance even in salty circumstances such as coast districts can be obtained by controlling the content of B and the content of one or more of P, Cu, Ni and Mo in relation the atmospheric salt content.
3) Compatibility with Earthquake Proofness
Further, the inventors have found that the sum of inclusions, particularly, the amount of A series and B series inclusions gives a significant effect on the toughness in the Z direction and the toughness in the Z direction can be improved remarkably by restricting the sum (dA+dB) value for the A series inclusion amount and the B series inclusion amount according to JIS G 0555 to 0.030% or less.
At first, the result of experiment conductive by the present inventors regarding the relation between the toughness in the Z direction and the amount of inclusions is to be explained.
Steels were prepared by melting while variously changing the forms and the amount of inclusions into steel plates of 60 mm thickness by hot rolling. Test pieces for microscopic observation and test pieces for Charpy impact shock in the Z direction (JIS No. 4 test specimen) were sampled from the steel plates, and the form and the amount of inclusions and the toughness in the Z direction (absorbed energy) were measured.
FIG. 1 shows a relation between the sum (dA+dB) value of the A type inclusions and the amount of B type inclusions according to JIS G 0555 and the Charpy absorbed energy (vExe2x88x925) in the Z direction at xe2x88x925xc2x0 C. In the Charpy impact test, ten specimens were used for each of the steel plates. Mean values and the minimum values for ten specimens are plotted respectively in the drawing.
As shown in FIG. 1, when the (dA+dB) value is 0.030% or less, absorbed energy of 47J or more at xe2x88x925xc2x0 C. and high toughness in the Z direction are shown including minimum values. On the other hand when the (dA+dB) value exceeds 0.30%, low values appear for the minimum value and also the mean value decreases below 47J.
FIG. 2 shows a relation between the dC value for the amount of C type inclusions according to JIS G 0555 and the Charpy absorbed energy in the Z direction at xe2x88x925xc2x0 C. (vExe2x88x925). In FIG. 2, the relation between the dC value and vExe2x88x925 is shown for the steel plates having the (dA+dE) value within range from 0.021% to 0.028%, which show high toughness in the Z direction.
It was not recognized from FIG. 2 that the dC value for the amount of C type inclusions give particular effect on the toughness in the Z direction.
In view of the above, the inventors have obtained the knowledge that control of the sum (dA+dB) value for the A type inclusions and the B type inclusions is important for improving the toughness in the direction of the plate thickness. Particularly, it has been found that the toughness in the direction of the plate thickness is improved remarkably by defining the (dA+dB) value to 0.030% or less.
FIG. 1 and FIG. 2 show the knowledge obtained from the coast weathering resistant steel materials and similar results have also been obtained for the flow rust reducing weathering resistant steel materials (FIG. 3 and FIG. 4).
This invention has been accomplished based on the findings described above.